A toner for use in the development of electrostatic latent image in electrophotography is prepared by a grinding process which comprises kneading a low melting resin with carbon black, a coloring agent such as dye pigment and a charge controlling agent, and then grinding and classifying the material. The toner prepared by the grinding process normally has an average grain diameter of 8 .mu.m to 12 .mu.m and a grain size distribution of 1 .mu.m to 25 .mu.m. In order to obtain grains having a narrower grain size distribution from this product, an accurate classification process is required. However, this classification process takes much time and gives a reduced yield, drastically raising the manufacturing cost and causing an economic problem. Further, in this kneading and grinding process, it is very difficult to homogeneously disperse additives such as coloring agent and charge controlling agent in the low melting resin. The poor dispersion of additives in individual grains deteriorates the properties of the toner. In particular, this causes uneven chargeability that results in unevenness in image quality or printing properties. Therefore, the relationship of the chargeability of individual grains with image quality, average grain diameter and grain size distribution becomes indefinite. This leads to variation of the properties of the toner from lot to lot, giving a poor reproducibility.
Various approaches have been attempted to eliminate the defects of the grinding process. As the most typical example of such an approach, a polymerization process for the preparation of an electrostatic development toner has been proposed.
An example of the polymerization process for the preparation of a dry toner is an emulsion polymerization process in an organic solvent as disclosed in JP-A-61-273552 and JP-A-61-273553 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). Both of the two proposals are disadvantageous in that the reaction requires the use of an organic solvent, the grain growth takes much time and the resulting grains have a broad grain size distribution.
An example of suspension polymerization process is disclosed in JP-A-61-22354 and JP-A-61-67039. In either process, as a suspension means there is used a homomixer which utilizes a shearing stress produced by a high speed rotary blade during suspension. This method is useful in shearing a polymerizable monomer as a main component but is poor in the production of grains having a narrow grain size distribution. The resulting product has a broad grain size distribution. This method is also disadvantageous in that it is difficult to homogeneously disperse a pigment and other various additives in the polymerizable monomer.
JP-A-63-113561 proposes the production of a dispersion by a high pressure homogenizer adapted to provide grains with a small grain diameter and a narrow grain size distribution and a method which comprises the treatment of a polymerizable monomer in such a dispersion. Referring to the operation mechanism of such a high pressure homogenizer, a pressurized solution to be treated is discharged through a controlled minute gap to a low pressure zone where it is then allowed to collide with an impact member such as impact ring. However, even this method cannot provide a suspension comprising grains having a grain diameter of 5 .mu.m to 16 .mu.m in a proportion of not less than 90%, making it difficult to prepare a product having a narrow grain size distribution with an average grain diameter of 4 .mu.m to 8 .mu.m. Therefore, in order to obtain a product with a small grain diameter and a narrow grain size distribution, a classification process that adds to the manufacturing cost is needed. A known prior art approach for providing a suspension with a smaller grain diameter and a narrower grain size distribution is disclosed in JP-A-5-156555. In this patent, a method is disclosed which comprises the production of a final suspension after pre-suspension. This method can exert an effect of producing a suspension with a smaller grain diameter and a narrower grain size distribution than the prior art suspension process. However, in order to prepare a suspension with a smaller grain diameter and a narrower grain size distribution, this method requires that the number of passes be increased. This causes an extreme increase in the output of finely divided grains. Such a product cannot be practically used as a toner unless these finely divided grains are removed. Thus, this method, too, requires a classification process.